I can't quite understand the concept of the extended marketing mix. What is it and What is the need to have it?

The extended marketing mix (7P's) is the combination of seven elements of marketing that aim to work together to achieve the objectives of a marketing strategy.

These 7 elements are: product; price; place; promotion; people; process and physical.

It is called a marketing mix because each element of the marketing mix is related to the others. 

by adding Physical, Process and People to usual 4P’s This is the so-called extended marketing mix.